The lower Valdarno, with the valley of important tributaries such as the Pesa, Elsa, and Era and in which, after Pontedera, the Arno flows into the Ligurian Sea.The middle Valdarno, with the plain including Florence, Sesto Fiorentino, Prato, and Pistoia.The Sieve's basin, which flows into the Arno immediately before Florence.The upper Valdarno, a long valley bordered on the east by the Pratomagno massif and on the west by the hills around Siena.The Val di Chiana, a plain drained in the 18th century, which until then had been a marshy area tributary of the Tiber.The Casentino, in the province of Arezzo, formed by the upper course of the river until its confluence with the Maestro della Chiana channel.The drainage basin amounts to more than 8,200 square kilometres (3,200 sq mi) and drains the waters of the following subbasins: It has many tributaries: Sieve at 60 kilometres (37 mi) long, Bisenzio at 49 kilometres (30 mi), Ombrone Pistoiese at 47 kilometres (29 mi), and the Era, Elsa, Pesa, and Pescia. With a length of 241 kilometres (150 mi), it is the largest river in the region. The river turns to the west near Arezzo passing through Florence, Empoli and Pisa, flowing into the Tyrrhenian Sea at Marina di Pisa. The river originates on Monte Falterona in the Casentino area of the Apennines, and initially takes a southward curve. High water marks of Arno river floods on Aug(left) and Novem(metal plate on the right).
